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FULLY DITIGAL - NO PUBLIC : PhD Defence Pim Willemsen | Biogeomorphology of Salt Marshes - Understanding the decadal salt marsh dynamics for flood defense

Biogeomorphology of Salt Marshes - Understanding the decadal salt marsh dynamics for flood defense

Due to the COVID-19 crisis measures the PhD defence of Pim Willemsen will take place online. 

The PhD defence can be followed by a live stream.

Pim Willemsen is a PhD student in the research group Marine and Fluvial Systems (MFS). His supervisors are prof.dr. S.J.M.H. Hulscher from the Faculty of Engineering Technology (ET) and prof.dr. T.J. Bouma from Utrecht University. 

The coastal zone is the most densely populated area globally and economic activity is high. 70% of the world’s megacities is located in this area, and the population is still growing. However, coastal protection is a prerequisite to work and live in this area, especially with predicted sea level rise and extreme storms becoming more frequent and severe. Salt marshes can contribute to coastal protection by having the ability to adapt to a changing environment, whereas conventional infrastructure like dikes and seawalls remain fixed over time. So by combining conventional coastal infrastructure and ecosystems in nature based flood defenses, coastal protection becomes more sustainable. The contribution of salt marshes to coastal protection by attenuating waves, depends on the size of the salt marsh and thereby the location of the salt marsh edge. The key question of this thesis is: What is the long-term (50 year) variability of the location of the salt marsh edge and what is the corresponding variability of the wave attenuating capacity of salt marshes?

The location of the marsh edge is restricted by two interacting factors: inundation time and bed level change. For vegetation establishment to withstand longer inundation stress, more stable bed levels are required so that plants are not disturbed. Vice versa, to withstand more dynamic bed levels that disturb plant growth, lower inundation stress is required, so that plants grow fast enough to recover from the stress. Moreover, the seaward growth and landward retreat of the marsh edge is driven by the magnitude of daily mild wave conditions and sediment availability. Periods of small waves and/or large sediment availability enable the marsh to grow seaward and vice versa. Short-term high waves affect establishing juvenile salt marshes, resulting in a retreating marsh edge. All processes resulting in retreat and expansion of the salt marsh edge, affect the wave attenuating capacity. Generally, the larger the cross-shore width of the salt marsh, the larger the wave attenuating capacity. Nevertheless, both artificial marshes and existing salt marshes contribute to the wave attenuation under a range of hydrodynamic conditions.